The present invention relates generally to keyboard assemblies and more particularly to a keyboard substrate that carries resilient domes each of which may be depressed to close a circuit.
Developments in solid state electronics have led to great reduction in the size of instruments such as calculators and related computational apparatus. In the kind of unit which employs a keyboard to permit input by means of finger tip push button depression, the keyboard assembly has tended to pose at least as much of a limitation upon size reduction of the overall unit as the electronic components which perform the interrelated operations. In furtherance of miniaturation of the keyboard assembly, one general approach implemented by several different manufacturers has involved the use of an electrical switch element in the form of a resilient metal dome. The marginal edge of the dome is in electrical contact with a first terminal carried by an insulating substrate, while the center of the dome overlies a second terminal also carried by the substrate. Upon depression of the central region of the dome into cntact with the second terminal, a connection is completed between the two terminals.
In its simplest form, such a dome is a smooth sector of a sphere. One drawback in the use of a simple sphere segment has been its lack of a sufficiently effective tactile feedback. That is, the user finds it difficult to detect through the sense of feel in this fingertips just when actual electrical contact has been made upon depression of the push button which flexes the domes. Thus, it becomes desirable that the dome exhibit what at least feels like a snap action. To this end, domes have been developed which include some sort of polygonal, usually triangular, shaping. In some cases, the marginal edge portion of the domes have been cut so as to define a polygonal shape. In another case, the marginal edge of the dome has been retained in circular shape but the body of the dome has been embossed in a pattern which has a generally triangular shape. In all these cases, the dome has been further embossed or deformed at each of the corners of the polygon so as to form a foot which rests upon a corresponding electrical terminal carried by the insulating substrate. A leading difficulty with these approaches has been a decided tendency for metallic fractures to develop at the edges of the feet leading to failure of performance of the dome prior to failure of any other component of the associated instrument.
Several procedures have been employed on substrates so that they carry the necessary conductive leads which are plated or otherwise printed thereon. In any event, however, in keyboard systems utilizing the resilient dome contact element, the foot or marginal peripheral area of the dome is in engagement with one conductive path and the apex or central area of the contact element is adapted to selectively engage, upon depression, a contact element disposed immediately beneath the apex of the contact element. The apex of a contact element upon depression will normally make a point type contact with the underlying conductive path, so with conventional dome shaped contact elements it is difficult to simultaneously engage a plurality of conductive paths with a single contact element. In numerous applications, the ability to simultaneously engage a plurality of conductive paths with a single dome shaped element is desirable and it is to this end that the present invention has been developed.
Accordingly, it is a primary object of the present invention to provide a new and improved contact element for keyboard switch assemblies which is adapted to simultaneously engage a plurality of conductive paths disposed immediately therebeneath.
It is another object of the present invention to provide a new and improved dome shaped contact element for a keyboard switch assembly wherein the contact element upon depression engages a relatively broad area of the substrate therebeneath as compared with conventional dome shaped contact elements which typically make only point contact with the underlying element.